The present invention concerns a method for welding two end portions of a coil onto the two electric contact pads or bumps of an integrated circuit or an electronic unit of small dimensions. The electric connection of the two end portions of the coil on the electric contact pads or bumps is generally effected by means of a thermode provided for this purpose.
A method for assembling a coil with an electronic circuit is known, in particular from U.S. Pat. No. 5,572,410. According to this method, an electronic circuit is provided, in particular a semiconductor integrated circuit and a coil core placed in proximity. Then, an insulated electric wire is automatically wound around the core, the two end portions of the coil being arranged so as to be superposed on the electric contact bumps of the electronic circuit (see FIGS. 1 and 3). Next, the two end portions of the coil located directly above the metal bumps are welded to the bumps by means of an automatic welding device which removes the insulation from the coil wire, i.e. the insulating sheath protecting the electric wire, simultaneously with the welding (column 3, lines 26 to 30).
A major drawback results from the assembly method described hereinbefore. Indeed, it has been observed that the implementation of the method described hereinbefore with integrated circuits causes relatively significant damage to the integrated circuit itself, in particular to the top layers, particularly the passivation layer.
Indeed, welding the ends of the insulated wire of the coil to the metal pads or bumps of the integrated circuit by thermocompression requires a certain pressure and a high temperature, for example of the order of 500xc2x0 C.
In the case of the present invention, it has been observed that the high temperature necessary to remove the insulating sheath and to simultaneously weld the electric wire onto the metal pads or bumps of the integrated circuit generates significant mechanical and thermal stress, due, in particular, to dilatation of the metal pads or bumps of the integrated circuit. The pressure and heat necessary for the method described hereinbefore thus generate microcracks in the integrated circuit which can damage it and make it unusable. Thus, the industrial yield with this method is relatively low.
The object of the present invention is to overcome the aforecited drawbacks by providing an efficient welding method which protects the circuit or electronic unit from mechanical or thermal stress damaging the circuit or electronic unit.
The present invention therefore concerns a method for welding two end portions of a coil onto two electric contact pads or bumps of an integrated circuit or an electronic unit of small dimensions, this method being characterised in that, prior to the step of welding the two end portions onto the two electric contact pads or bumps, a preliminary step is provided, consisting in removing at least partially the insulating sheath from the electric wire forming the coil in the regions of the two end portions provided for welding. Preferably, the localised removal of the insulating sheath is effected by a heat supply used to melt or sublimate the insulating sheath at the locations of the two end portions provided for welding the coil to the integrated circuit or electronic unit.
As a result of the welding method according to the invention, it is possible to weld the two ends of the electric wire forming the coil to the integrated circuit or electronic unit with a relatively low temperature, in particular less than 200xc2x0 C. Indeed, the removal of the insulating sheath can be effected at a high temperature of the order of 500xc2x0 C., whereas the temperature necessary for welding the electric wire itself onto the metal pad or bumps of the circuit or electronic unit requires a much lower temperature. Moreover, a relatively low welding pressure is necessary in these conditions. In a particular embodiment of the invention, the ends of the electric wire of the coil are welded using an ultrasound head which leaves the metal pads or bumps at a temperature close to the ambient temperature.